The present disclosure generally relates to a clear or substantially colorless overcoat composition that may be used for overcoating, for example, ink based images and xerographic images. The overcoat composition described herein is a clear solid ink which comprises one or more ultraviolet (UV) absorbing additives. Also described is a method and systems for using and detecting the overcoat composition in connection with a substrate, for example, a substrate that includes printed images thereon.
Wax based inks, sometimes known as phase change inks or solid inks, are used to form digital images on paper using heated piezoelectric or acoustic ink jet heads. The ejection of an ink drop by the print heads is electronically controlled. In embodiments, the hot drop partially cools upon hitting an intermediate surface, often an aluminum drum. The complete image is assembled on the intermediate surface and then transferred to paper and fixed thereon with a combination of pressure and heat resulting in a solid ink, or wax-based ink print. Alternatively, the wax-based image can be printed directly to the substrate. The direct-to-paper image is also exposed to a combination of pressure and heat to fix the image to the substrate.
Known methods of protecting ink or toner-based images include applying an overcoat composition to the substrate. Both surfaces, however, can be difficult to subsequently coat. In addition, the coating of prints made with solid ink or wax-based ink can be especially difficult due to the fact that the robustness of such prints is relatively poor when compared to inkjet or xerographic prints. This fact raises a significant challenge in applications used in the advertising mail, or “direct-mail,” market. The conventional approach in direct-mail application is to overcoat the text with clear ink text image that is registered to the visible image, which requires printing and registration of the clear color in the same manner as that used with other colors (e.g., cyan (c), magenta (m), yellow (y), black (k)). With colors, the printing and registration is achieved with certain systems, such as for example, an Image-on-web-array (IOWA) inline scanning sensor, which are configured to detect visible colors. The problem with using the clear ink with such systems, however, is that the clear ink is undetectable since it is clear. Thus, the current systems require a clear solid ink that is detectable.
To address the above issues, previous proposed methods used either fluorescent or infrared (IR) sensitive materials in the clear overcoat ink compositions, as disclosed in U.S. Pat. No. 7,690,746, which is hereby incorporated by reference. However, fluorescent materials have several disadvantages, such as high cost—particularly of clear fluorescent dyes, poor contrast on white bright paper, and poor thermal stability. IR dyes also have disadvantages, such as poor solubility in low polarity media like solid ink base. IR dyes are also never truly invisible in normal light conditions which renders the clear overcoat composition visible. Alternatively, U.S. patent application Ser. No. 12/757,415 to Michelle N. Chretien et al., filed on Apr. 9, 2010, which is hereby incorporated by reference, proposes the use of fluorescence measurement of paper substrates through the use of a clear UV gel ink. However, this method does not work with clear solid ink overcoat compositions because the solid ink does not have any absorption when exposed to black light, a currently easily available UV light.
Thus, there remains a need for an overcoat composition that can be used for protecting solid ink and toner-based images, which provides overcoat coating properties including, for example, thermal and light stability, scratch resistance, and smear (or rub) resistance to toner-based images and ink-based images, particularly in commercial print applications, and which can also be detected by scanning sensors used in current printing and registration systems.